This machine was water damaged and the battery management system "BMS" which is located inside the battery shorted causing damage to the SMC data lines that communicate with it. The system management controller "SMC" integrated circuit because of this damage was unable to properly over come a 2k Ohm pull up resistor to properly pull down the data line and create data signals to communicate with the BMS and U7100. These packets were visible but were not properly pulled to ground. The SMC was fully functional otherwise and the machine did power on and operate. I go into detail of the charging circuit and how it works. Covering not only the communication between the SMC, BMS and U7100 and show how these should look on an oscilloscope. This is the first time I have seen this fault. I did save the defective SMC and may make future videos with this part. This machine helped me dive into a area of the board I have not had to troubleshoot in great detail and gave me a much better understanding of these systems and how they work. This was not the kind of repair where you could see damage on the board and just fix it. You had to read the schematics and figure your way through the system. The SMC chip is not something you change to "just see" if it fixes your problem. It is a last resort and you want to prove it is defective through troubleshooting first. I enjoy challenging faults that force me to expand my knowledge of these boards. This was one of those repairs. These circuits and communication data lines are basically the same on all MacBooks regardless of model so hopefully this can help you in the future.